<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>清除内外边距</title>
    <style>
        /* 每个项目开头必须写的 */
        * {
            padding: 0px;
            /* 清除内边距 */
            margin: 0px;
            /*清除外边距*/
        }

        span {
            background-color: red;
            margin: 0px 20px;
        }
    </style>
</head>

<body>
    <span>1111111</span>
    <ul>
        <li>123456</li>
    </ul>
    <span>1aiicnainciorcoac</span>
</body>

</html>